1. Field of the Invention
The invention relates generally to the field of Positron Emission Tomography (PET). More particularly, the invention relates to scintillation radiation detectors in PETs. Specifically, a preferred implementation of the invention relates to the manufacture of the scintillation radiation detectors.
2. Discussion of the Related Art
Positron emission tomography (PET) is a technique for measuring the concentrations of positron emitting radioisotopes within the tissue of living subjects and using those measurements to form images of the internal tissues. PET may require a cyclotron as an on-site source of short-lived positron-emitting isotopes. The isotopes are injected into the patient along with a glucose-related compound, and the positrons collide with electrons in body tissues to produce photons. The photons are tracked by a tomographic scintillation detector, and the information is processed by a computer to provide both images and data on blood flow and metabolic processes within the tissues observed.
The tomographic scintillation detector is a vital part of the PET. Without it, imaging of the tissues cannot take place. The detectors are arranged into arrays. Each array is a matrix of scintillation crystals, each optionally rectangular in shape. When a gamma or other radiation particle strikes a crystal detector element in the array, light is emitted. The light signal is distributed to four or more photosensors, as shown in FIGS. 1–3. The amount of light going to each of the photosensors from this stimulated detector, a transparent crystal, is controlled by either some light partition or coupling between the crystals, or by a light guide system between the crystal array and the photosensors. The four or more photosensors turn the light signals into proportional electronic signals. The relative magnitude of the electronic signals from the four or more photosensors is used to deduce the position of the scintillating crystal. This type of position-sensitive detection system is widely used in radiation imaging. The performance of the system is determined by the accuracy of deducing the position of the scintillating crystal. The accuracy of decoding the position is in turn determined by the design of the light-partition, light coupling or light-guide.
As shown in FIG. 2, one traditional way of building a detector array with the optimal light distribution is as follows: a solid crystal block that is cut with unequal saw-cut depth in both the transaxial and axial dimensions. This process is satisfactory for the larger crystal elements used in lower resolution cameras but not for the very high resolution, small crystal elements, cameras. One reason is the substantial loss in coincidence detection efficiency for very small detectors due to the width of the grooves created by the saw blade, asPET coincidence detection efficiency=(detector efficiency)2=(detector transaxial packing fraction×axial packing fraction)×(detector transaxial packing fraction×axial packing fraction).
If the detector pitch is 1.7 mm, which couples to a saw-blade groove of 0.4 mm (typical), the detector packing fraction would be (1.7−0.4)/1.7=0.76 along both the transaxial and axial dimension. Hence, for the case of a PET camera, the coincidence efficiency can be (0.76×0.76)2=0.33. In other words, 67% of the coincidence events will be lost by the saw cut for a detector pitch of 1.7×1.7 mm.
As shown in FIG. 3, a second way of making position-sensitive detectors is to put individual crystals, each optically isolated by painting or masking all the 4 side surfaces, onto a light-guide plastic block that has unequal grooves cut into it. In this case, the unequal grooves are in the light guide instead of the crystal block. The crystals need to be individually cut and polished. Then the individual crystals are placed and glued onto the light guide manually or by a robotics device. In either case, there is a gap between crystals for the clearance of the tweezers or robotics fingers that grab and place the crystals onto the light guide. However, the gaps between the crystals also reduce detection sensitivities. Furthermore, since individual crystals have to be cut and polished mechanically or chemically, this process is more labor intensive.
A problem with manufacturing individual crystals for the detectors is in the actual placement of the individual crystals into the array. Mechanical precision is important in the manufacturing of imaging detector systems because tens of thousands of scintillation crystals are closely packed together. These crystals are often very small: 1–5 mm. The buildup of tens of thousands of small mechanical errors (i.e., 0.1 mm per detector) can be a significant error relative to the small sizes of the crystals, which can place some crystals in the detector arrays too far from its expected position, which can degrade the imaging accuracy. What is needed is a method of manufacturing that can decrease the total sum of mechanical errors caused during the making of the detector arrays.
Another disadvantage of conventional approaches has been the high cost of manufacturing each crystal individually. Therefore, what is also needed is a solution that meets the above-discussed requirements in a more cost-effective manner.
Yet another way of creating detector arrays is to cut out channels in the scintillation block material and then covering the channels and grooves with a light reflecting material. This is an improvement over the previous methods because it allows the interval between respective channels in a detector array and enhances the arrangement accuracy of the respective channels.
A problem with cutting uneven grooves into the scintillation material has been that the depth of the cut in the crystal is very deep at the ends of the block, and the small amount of material left to connect the channels is easily breakable. If the material breaks, the whole detector crystal block is wasted and unusable. Therefore, what is required is a solution that is less prone to breakage and that will not render the whole detector useless when a portion of it fails or breaks off.
For the manufacture of circular detector arrays, additional grinding of the scintillation crystals is needed to eliminate the crystal overlap present in a circular detector arrangement. This additional process may be time and cost extensive, as it may require the grinding of each individual crystal. For scintillation crystals blocks created by cutting groves into the blocks, additional grinding may increase the odds of creating a defective crystal block by breaking off crystals at the ends of the blocks.
Heretofore, the requirements of a more durable scintillation detector array, decreased mechanical errors in the arrays, decreased gaps between crystals, and decreased cost of capital and time in the manufacturing of the detector arrays have not been fully met. What is needed is a solution that addresses some or all of these requirements.